Most modern color picture tubes have inline electron guns. An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and to direct those beams along convergent paths to a point or small area of convergence near the tube screen. Inline electron guns have a beam forming region and a main focus lens and may also include a prefocus lens. The beam forming region usually comprises three cathodes and three consecutive electrodes. A prefocus lens may comprise two or three electrodes. The main focusing lens is usually formed by two spaced electrodes.
Characteristically, the two outer electron beams, usually associated with red- and blue-emitting phosphors in most color picture tube inline electron guns, move horizontally as focus voltage is varied. This focus voltage-related sensitivity characteristic of a tube is commonly referred to in the art as its differential horizontal FRAT or FRAT-X R/B. Typically, FRAT-X R/B is defined as the movement of a beam landing on the screen of the tube, measured in millimeters, for a focus voltage change of 2 kV. The magnitude of FRAT-X RIB depends on dimensions and spacings of electron gun electrodes and is usually non-zero and variable from gun to gun. Variation in FRAT-X R/B magnitude poses several problems. First, to the extent that the focus voltage used in a particular receiver chassis is different from the focus voltage used when a yoke was originally attached to the tube, any magnitude of FRAT-X R/B will cause static convergence errors over the screen area. Secondly, a tube-to-tube difference in FRAT-X R/B is an indication of different inter-beam spacings in the main lenses of the tube electron guns, which creates grouping differences in the tubes. Both are potential problems to a tube maker, who generally seeks a small magnitude and controlled value of FRAT-X R/B.
By design, one can usually achieve an acceptably small value of FRAT-X R/B by using known techniques of aperture offsets and/or thickness variations in the electrodes. However, there are families of electron guns which use many common electrodes, but have slightly different FRAT-X R/B due to non-common electrodes, thus causing grouping differences when used in the same tube. These grouping differences, caused by FRAT-X R/B differences typically less than 1 mm/2 kV, can be corrected in a particular tube by means of known screen and lighthouse adjustments (i.e., commonly known as "Q" and "X" adjustments).
A relatively new problem has emerged, namely, the fabrication of tubes of the same size, but having different electron gun types, where the tube manufacturer cannot adjust the photoscreening lighthouses for each gun type and must live with a compromise situation in terms of grouping errors for all gun types. In such a case, a difference of 1 mm in FRAT-X R/B between tubes may be unacceptable.